how to upgrade lg-d820(e) nexus 5 hammerhead to marshmallows?

[Muhammadsaqib11]

model: D820(e) Hammerhead Nexus 5
OS version: 5.0
baseband: m8974a-2.0.50.2.21
kernel: 3.4.0-g323bbd9
[email protected] #1
thu oct 9 05:57:15 utc 2014
build: lpx13d

fastboot:

max-download-size:         0x40000000
version-hardware: rev_11
version-baseband: M8974A-2.0.50.2.21
version-bootloader: HHZ12d
version-cdma: N/A
variant: hammerhead D820(E) 32GB
serialno: 07977be40068f5b5
carrier: None
secure-boot: yes
unlocked: yes
product: hammerhead
partition-size:aboot: 80000
partition-type:aboot: emmc
partition-size:boot: 1600000
partition-type:boot: emmc
partition-size:recovery: 1600000
partition-type:recovery: emmc
partition-size:system: 40000000
partition-type:system: ext4
partition-size:userdata: 6ce9fa800
partition-type:userdata: ext4
partition-size:cache: 2bc00000
partition-type:cache: ext4
partition-size:persist: 1000000
partition-type:persist: ext4

this is my 1st case......a little hesitate, nurvious so much...
will anybody help me to upgrade it to marshmallows 6.0 or above?

i deal a lot of cell in past but those was bricked position therefore i couldnt deal them positivly.
now this devices is working fine only want to upgrade it.
please guide me how to upgrade and how to match firmware?

thanks a lot